Aluminum oxide semi-conductors



United States Patent ALUMINUM OXIDE SEMI-CONDUCTORS William A. Miller,Port Jefferson, N. Y., assignor to Radio Corporation of America, acorporation of Delaware No Drawing. Application September 23, 1948,

Serial No. 50,883 Y 7 Claims. (Cl. 29-182-1) This invention relates toimprovements in that class of materials known generally assemi-conductors and having a negative temperature coefiicient ofresistance. The resistance of this class of materials is relatively highbut not nearly as high as that of materials classed as good insulators.

In certain applications of semi-conducting materials, it is desirable toemploy a form having a low temperature coefficient of resistance. Thepresent invention relates more particularly to materials of this nature.

One object of the present invention is to provide improved materialshaving relatively high resistance and a relatively low temperaturecoeflicient of resistance.

Another object of the invention is to provide a novel.

product comprising aluminum oxide and aluminum, which product has anegative temperature coefiicient of resistance.

Another object of the invention is to provide improved semi-conductingmaterials which can be operated at high temperatures.

These and other objects will be more apparent and the invention will bebetter understood from the following specification.

In general, the products of the present inventi n are made by combiningcertain proportions of aluminum and aluminum oxide at sinteringtemperatures in the presence of a non-oxidizing atmosphere. By anonoxidizing atmosphere is meant a vacuum, a reducing at mosphere or aninert atmosphere. It is preferable to exclude water vapor from theatmosphere.

Aluminum, itself is, of course, a good conductor of electricity whilealuminumoxide is normally a good insulator. If so much aluminum is addedto the aluminum oxide that a continuous conducting path around theinsulating particles is provided by the aluminum, the mixture would thenbecome a fairly good conductor. However, this type of composition doesnot come within the scope of the present invention. It has been foundthat aluminum will disperse freely throughout a body of aluminum oxide.It is not possible to mix any randomly selected metal with a randomlyselected ceramic material and have the metal disperse freely throughoutthe ceramic. Also, in those compositions in which there is freedispersal, the resulting product is not always a semi-conductor having anegative temperature coefficient of resistance. It, therefore, appearsto be an individual problem as to whether any particular metal and aparticular ceramic will produce the desired type of product.

Example I One part of aluminum was mixed with three parts aluminum oxideand the mixture molded into a cylinder one-sixteenth inch in diameterand one-fourth inch long.

The cylinder was fired in vacuo at about l,500 C. for

,. lce

Example II A cylinder similar to that above described was made upcontaining one part aluminum to twenty-five parts aluminum oxide andfired at the same temperature and for the same time as the material inExample I. When tested under similar conditions, the product had aresistance or about seventy-five ohms.

The complete range of proportions of aluminum oxide to aluminum has beenfound to be from 3 to l toabout 40 to 1 by weight.

The firing temperatures may range from about 1,450" C. to temperaturesjust below the melting point of aluminum oxide (2,050 C.). The time offiring varies inversely with firing temperature. Times of twenty minutesup to one and one-ha1f hours have proved satisfactory depending upon thetemperature of firing. There does not appear to be any critical upperlimit of time after the product has once become stabilized. Furtherheating neither causes deterioration of the product nor improvement. Ifvacuum heating is used, the vacuum must be good enough so that allappreciable amounts of oxygen are excluded. The inert atmosphere may beany of the usual inert gases such as helium, nitrogen or argon.

Since aluminum vaporizes readily in a vacuum, the product may beprepared by first forming a compressed body of aluminum oxide and thenmaintaining the com.- pressed body in an atmosphere of aluminum vaporuntil sufficient diffusion has taken place of the metal throughout theceramic.

Another way to combine the two substances is to immerse a compressedpellet of the ceramic in a bath of molten aluminum.

The principal advantages of the improved materials of the presentinvention are:

First, that they are highly refractory and, therefore, eminently suitedto those uses where large amounts of power must be dissipated at hightemperatures, and

Second, that they are useful where it is desired to use a materialhaving a very low negative temperature coefiicient of resistance.

There has thus been described a new and useful type of semi-conductingmaterial comprising aluminum and aluminum oxide ceramically combined. Itis desired that the invention be limited only as defined in the appendedclaims.

I claim as my invention:

1. A semi-conductor having a negative temperature 00- efiicient ofresistance, consisting essentially of a compact porous body of sinteredaluminum oxide and aluminum, said body being made by diffusing aluminurmmetal homogeneously throughout the pores of a compact body of aluminumoxide at sintering temperatures of said oxide above about l,450 C. butbelow the melting temperature of said oxide and in a non-oxidizingatmosphere, and the ratio by weight of said oxide to said aluminum beingwithin the range of about 40 to 1 and about 3 to l.

2. A material according to claim 1 in which said ratio is 25 to l. v

3. A material according to claim 1 in which said ratio is 3 to 1.

4. A method of making a semi-conducting material comprising diffusingaluminum metal homogeneously throughout the pores of a compressed bodyof aluminum oxide, that is, above about 1450 C. at sinteringtemperatures of said oxide but below the melting point of said oxide andin a non-oxidizing atmosphere, the ratio by weght of said oxide to saidaluminum being within the range of about 40 to 1 and about 3 to l.

5. A method according to claim 4 in which the oxide and the aluminum aremixed together before the oxide is sintered.

6. A method according to claim 4 in which the oxide is first formed intoa compressed body, simered and their immersed in molten aluminum.

7. A method according to claim 4 in which the oxide is first formed intoa compressed body, sintered and then exposedto an atmosphere of aluminumva or.

References Cited in the file of this patent UNITED STATES PATENTS 4Bethel Oct. 17, 1933 'MAfifiiif 061. 28, 1941 Goldschmidt et a1 May 19,1942 Preis Apr. 27, 1943 Colbert et a1. Dec. 16, 1947 Alexander 2.22;;July 6, 1948 McKinley Mar. 1, 1949 Hend'ren s s== s Mar. 29, 1949FOREIGN PATENTS Great Britain 11113: 8, 1920 OTHER REFERENCES Metalindustry, May 14, 1948, pa es 405-407.

1. A SEMI-CONDUCTOR HAVING A NEGATIVE TEMPERATURE COEFFICIENT OFRESISTANCE, CONSISTING ESSENTIALLY OF A COMPACT POROUS BODY OF SINTEREDALUMINUM OXIDE AND ALUMINUM, SAID BODY BEING MADE BY DIFFUSING ALUMINUMMETAL HOMOGENEOUSLY THROUGHOUT THE PORES OF A COMPACT BODY OF ALUMINUMOXIDE AT SINTERING TEMPERATURES OF SAID OXIDE ABOVE ABOUT 1,450* C. BUTBELOW THE MELTING TEMPERATURE OF SAID OXIDE AND IN A NON-OXIDIZINGATMOSPHERE, AND THE RATIO BY WEIGHT OF SAID OXIDE TO SAID ALUMINUM BEINGWITHIN THE RANGE OF ABOUT 40 TO 1 AND ABOUT 3 TO 1.